1. Field of the Invention
The invention relates to a rotor for an electrical generator and to an electrical generator incorporating such a rotor.
In particular the invention relates to a rotor for a low speed generator and to a low speed generator incorporating such a rotor, that is to say a generator in which a driving force is applied to a power means, and in particular a rotating power means such as a turbine, which is mechanically coupled to cause rotation of a rotor either by direct drive or via a low-ratio gearing.
The invention relates especially to the provision of large scale wind turbine generators and marine current turbine generators, and the prior art is discussed and examples of the invention are given in the context of the former in particular. However, it will be understood that the invention finds potential application generally in electrical generators where rotors experience high torque.
2. Description of Related Art
Most of the first commercial wind turbines used electrical generators that were adapted from general purpose induction motors. It was common practice to connect the turbine rotor shaft rotating at typically 50 rpm or less via a multi-stage gearbox providing high ratio gearing to a generator typically rotating at approximately 1500 rpm. However, multi-stage gearboxes are complex, heavy, costly, require maintenance and there have been reliability issues in wind turbine applications.
As the wind power industry has grown and the typical power ratings of turbines has been increased, new generator types have been developed to meet the specific demands of the sector. Recently, direct-drive generators, which eliminate the gearbox entirely, have emerged and captured a significant proportion of the market. These generators are designed to provide the required electrical output whilst rotating at the same speed as the turbine, thereby making the gearbox unnecessary. This simplifies the mechanical arrangement, potentially reducing costs and maintenance outage times.
In conventional direct drive generators of typical design, a large diameter bladed turbine, typically two or three blades, is axially spaced from and directly coupled via the turbine rotor shaft to a smaller diameter rotor. Direct-drive generators need to have a large diameter to compensate for the low rotational speed of the turbine and retain adequate peripheral speed of the generator rotor. Often the axial length of the generator is quite small. Direct-drive generators are therefore characterised by a disc-like appearance.
There is a general desire to introduce larger turbines to increase capacity and efficiency, particularly in relation to the introduction of large offshore wind farms. Existing direct-drive turbines will become impractical if simply scaled up from existing designs because of problems associated with manufacture, weight and transportation of components of such extreme dimensions.
Wind turbines of 2 MW rating are now available commercially and much larger generators are envisaged, mainly for the emerging offshore market. However, for example, a 5 MW direct drive generator would need to be approximately 15 meters in diameter and would be extremely heavy.
There are several problem areas arising from the size and weight of these generators. These include:    (i) the need for very large bore machine tools    (ii) high roofed workshops with large capacity cranes are needed    (iii) transportation of large out-sourced parts for assembly, or the finished generator. This is even more problematic for the export market    (v) installation at the tower head    (vi) tower head weight impact on other aspects of the wind turbine design
U.S. Pat. No. 6,064,123 describes an alternative approach for wind turbine generators to the typical design of a larger diameter bladed turbine axially spaced from a smaller diameter rotor with or without a gearbox in between. In U.S. Pat. No. 6,064,123, the several large wind turbine blades which are normally used to extract power from the wind are replaced by a rotatably mounted central hub, a rim concentric with the hub and a plurality of blades disposed between the hub and the rim. The rotor of the generator is incorporated into this turbine structure by having a plurality of magnets disposed on the rim for generating current in the stator. This structure means that the rotor rim diameter is similar to the diameter of the blade arrangement.
Whilst this structure gives high peripheral speed relative to an equivalent conventional direct drive design, with the associated benefits of high peripheral speed, the extremely large structure with combined purpose exacerbates some of the above problems and creates problems of its own, and as a result fails to exploit or allow many of the other benefits identified with conventional direct drive design (for example in the use of two or three blades).
The heavy iron cores associated with conventional design direct drive generators also serve to exacerbate problems, in particular in relation to weight, efficiency losses, magnetic forces during assembly and air gap forces.
As the skilled person will appreciate, many of the above problems also arise in relation to other situations where large rotors, and in particular large disc rotors, are conventionally employed. In particular, similar problems arise in relation to the design of marine current turbine generators and the like.